3 o6 GENERAL SCIENCE 



the rock caused by the changes in temperature made the 

 rock crumble into fine particles. The freezing of water in 

 the rock greatly assisted this crumbling process. 



Small quantities of this disintegrated rock collected in 

 various places and made it possible for the lowest plant 

 forms to begin life. The decomposed rock contained the 

 necessary chemical elements for primitive plant growth. 



BENEATH THE SOIL is ROCK 



As these simple plant forms grew to maturity and died, 

 they added decaying plant matter to the forming soil. 

 This decaying matter, as soon as it became soluble, served 

 as food for other plants. Then came a process of plant 

 changes, or the evolution of plants. As plant food be- 

 came more abundant, the simple plants grew larger and 

 more complex in their structure. They decomposed the 

 rock with their roots, took food from the air with their 

 leaves, and then in turn died and decayed; thus the soil 

 was increased in depth and richness. This process con- 

 tinued for ages, and now the larger part of the land sur- 

 face of the earth is able to support plant life of some type. 

 Since man has been growing plants for food many rich 



